The instant invention relates to optical bistable devices, or optical flip-flops, for use in optical logic devices.
An example of a known optical flip-flop is taught by Jewell in U.S. Pat. No. 4,573,767, issued Mar. 4, 1986. Jewell teaches a flip-flop comprising a pair of optically aligned non-linear Fabry-Perot etalons. The transmissivity of each etalon is manipulated by the impingement thereupon of one of a pair of pulsed laser beams. The varying transmissivity of the etalons in turn controls a signal beam directed therethrough from a light source in optical alignment therewith. Unfortunately, as with other prior art optical flip-flops employing media having variable refractive indices or absorption coefficients, the transmitted light signal undergoes appreciable attenuation upon passage through the etalons, thereby requiring periodic reamplification of the transmitted signal. In his U.S. Pat. No. 4,630,898, issued Dec. 23, 1986, Jewell teaches an etalon optical logic gate which seeks to minimize attenuation of the transmitted light signal but does not eliminate it.
It is noted that the instant invention utilizes optical phase conjugation in order to generate a bistable reference beam from which an output beam is derived. The operating principles and general construction of one form of phase-conjugating means-- phase-conjugating mirrors--are discussed in V. Shkunov et al., "Optical Phase Conjugation", Scientific American, December, 1985, pp. 54-59. Briefly, a phase-conjugating mirror is a mirror which, by the process of optical phase conjugation, reflects impinging light back along its incident path in a "time-reversed" fashion. One type of phase-conjugating mirror uses a method known as four-wave mixing wherein a pair of "pump" beams and a probe beam impinge upon a non-linear medium. The probe beam interacts with one of the pump beams to distort the medium, while the other of the pump beams interacts with the resulting distortions to produce a "time-reversed" replica beam which travels back along the incident path of the probe beam. A fourth beam is generated within the phase-conjugating mirror by such interaction and exits the phase-conjugating mirror in the original direction of travel of the probe beam. See the Shkunov article referred to hereinabove, and D. Pepper, "Applications of Optical Phase Conjugation", Scientific American, January, 1986, pp. 74-83. Additionally, it is noted that some types of phase conjugators are capable of spontaneously generating an autocollimated, continuously reflecting light beam with a surface of sufficient reflectivity exposed thereto.